1. Field of the Invention
The present invention relates to an antiglare film, an antireflective film, a polarizing plate and an image display device.
2. Description of the Related Art
In a variety of image display devices, such as a liquid crystal display (LCD), a plasma display panel (PDP), an electroluminescent display (ELD) or a cathode ray tube display (CRT), antiglare film or antiglare antireflective film is used at each individual display surface for the purpose of preventing a contrast drop caused by reflections of extraneous light from the display surface and surroundings' reflection in the display screen. These image display devices are widely used in office and home environments, and an improvement in antiglare capability, or the capability of preventing indoor fluorescent lamps and viewers from being reflected in the display screen, and a further improvement in display contrast in well-lit rooms are required of them (see JP-A-2004-170901 for example).
By addition of light-transmitting particles to its antiglare layer, an antiglare film can utilize two functions, namely an antiglare function attributed to the scattering of light from asperities that the light-transmitting particles form on the antiglare layer surface (surface scatter capability), and a light-scattering function arising from a refractive index difference between each light-transmitting particle and light-transmitting resin in the antiglare layer (internal scatter capability).
In the case of imparting the antiglare function arising from the surface scatter capability, there however occur problems that an image display screen looks whitish because of its surface scatter capability, resulting in deterioration of a deep-black feel, and glitter traceable to lens effects produced by asperities on the layer surface becomes worse.
The internal scatter capability is utilized for the purpose of improving glitter and viewing angle characteristics of contrast, but when the scattering of light is excessive, there occurs a drawback of lowering display contrast. On the other hand, when the scattering of light is too little, there occurs a problem that glitter becomes worse. Therefore, moderate scattering of light on the inside is required. In order to impart a moderate internal scatter capability, a difference in refractive index is required to be made between each of the light-transmitting particles and the light-transmitting resin.
When a difference in refractive index is made between each of the light-transmitting particles and light-transmitting resin used in the antiglare layer for the purpose of imparting the internal scatter capability, it is preferred that the resin constituent in the light-transmitting particles be made different from the resin constituent in the light-transmitting resin. However, the use of different resin constituents causes a problem that aggregation of light-transmitting particles is apt to occur in light-transmitting resin because of weak affinity between the light-transmitting particles and the light-transmitting resin; as a consequence, surface unevenness become too great, and thereby deterioration of denseness of black is caused.
On the other hand, it is also possible to use antireflective film at the topmost surface of which a low refractive index layer is provided as a thin-film layer about 100 nm in thickness and antireflection is performed by optical interference of the low refractive index layer. And antireflective films offered with the intention of further lowering the reflectance include those of multilayer thin-film interference type which prevent reflection by optical interference of multiple layers, such as an antireflective film of two-layer thin-film interference type which has a high refractive index layer between a transparent support and a low refractive index layer, and an antireflective film of three-layer thin-film interference type which has a medium refractive index layer and a high refractive index layer provided in order of mention between a transparent support and a low refractive index layer. In particular, the antireflective film of three-layer thin-film interference type is suitable for the purpose of preventing reflection over a wide wavelength range and thereby achieving low reflectance while minimizing reflected colors.
As the thin-film layers (a high refractive index layer, a medium refractive index layer and a low refractive index layer) used in an antireflective film of multilayer thin-film interference type, multilayer film formed as lamination of transparent thin-film layers of metal oxides has been widely used so far. In general, formation of transparent thin films of metal oxides has been carried out using e.g. a chemical vapor deposition method (CVD) or a physical vapor deposition method (PVD), notably a vacuum evaporation method or a sputtering method as one type of physical vapor deposition method.
However, formation of transparent thin films of metal oxides by use of a vapor deposition or sputtering method is low in productivity and not suitable for mass production. Accordingly, wet film-forming methods, notably a method of forming film by coating, have been offered.
For the purpose of improving surroundings' reflection and denseness of black, it is favorable to form an antireflective layer of two-layer or three-layer thin-film interference type on an antiglare layer. However, when it is tried to coat an uneven surface of an antiglare layer with thin films on a nanometer scale, there may be cases where, since the uneven surface of the antiglare layer gives rise to uneven coating and repelling, the films formed cannot have uniform thickness and optimum optical interference does not occur as it is designed; as a consequence, reflectance is not lowered. Accordingly, it is difficult to achieve both low reflection and high antiglare properties at the same time.
On the other hand, there is an introduction to the technique of incorporating core/shell particles in a hard coating layer (JP-A-2003-183586). Although it discloses the art of lowering the density of cross-links in a shell portion by specifying a glass transition temperature of the shell, JP-A-2003-183586 has no disclosure about impartment of a light-scattering capability without roughening the antiglare layer surface for the purpose of providing on an antiglare layer a uniform antireflective layer of thin-film interference type, so further improvements are required.
Additionally, when it is tried to impart the internal scatter capability by use of light-transmitting particles in an antiglare layer, it is necessary to make a refractive index difference between each light-transmitting particle and light-transmitting resin of the antiglare layer, so light-transmitting particles formed of a material different from the light-transmitting resin are used as the particles causing internal scattering of light. In this case, however, since the affinity between the light-transmitting particles and the light-transmitting resin is weak, the light-transmitting particles cause aggregation in the light-transmitting resin and form uneven spots on the layer surface. As a result, there occur problems that the antiglare layer suffers deterioration in a feel of denseness of black and the antireflective film formed thereon cannot have uniform thickness because of uneven coating and repelling of each thin-film layer, and cannot achieve a reflectance drop.